Patent GB-584313-A (GB)
Title: Voice frequency ringing receiver
IPC: H04Q1/446
Filed: 1944-04-14
Abstract: 584,313. Voice immunity circuits. STANDARD TELEPHONES & CABLES, Ltd. April 14, 1944, No. 6900. Convention date, May 12, 1943. [Class 40 (iv)] . In systems in which signals and speech are transmitted over a common path, the frequency of the signals lying within the speech range, a receiver responsive only to the signals comprises means for receiving all frequencies and directing them to two parallel paths, one channel passing only the signal frequency while the other passes all other frequencies and means for combining their two outputs differentially to operate a signal relay when the output from the first channel is substantially greater. The receiver is described as applied to a system using 1000 c /s. frequency for ringing. Input from line, received through transformer 401, Fig. 1, is amplified in V1. Part of the output passes through a narrow band filter 407 and is rectified in V2. The remainder of the input, being the full range of frequencies except for the 1000 c/s. band, is tapped off potentiometer 405, 406 and rectified in V3. The anodes of V2 and V3 are connected to the grid and cathode respectively of amplifier V4, whose anode feeds relay 107. If only 1000 c/s. ringing tone is being received, this passes wholly through filter 407 and valve V2, whose increased impedance on rectification raises the potential of the grid of V4 with respect to the cathode. V4 then passes current and relay 107 operates. During receipt of normal voice frequencies any 1000 c/s. components are again rectified in V2 while all other frequencies are rectified in V3. The amplitude of 1000 c/s. components is very small compared with the sum of all other frequencies and, on rectification, the impedance of V3 rises much more than that .of V2. The potential of the cathode of V4 rises, no anode current flows and relay 107 remains unoperated. A time delay action by condenser 415 and resistances 410, 412 overcomes the possibility of relay 107 being operated by random 1000 c/s. transients. The differentiating effect of the circuit is increased by connecting the anode of V2 to the control grid of V3 over condenser 417. In order to prevent chattering of relay 107 if the 1000 c/s. tone is modulated by a low frequency, the screen grids of V2 and V3 are connected to the anode of V1. The modulation appears in the screen current of V2 and is fed back to path B and the control grid of V3 so that the resultant outputs of V2 and V3 are in antiphase. As open to inspection under Sect. 91, the Specification also describes a system for transmitting the ringing tone over the line. At the transmitting end the operation of the ringing key 102, Fig. 1 (Cancelled) passes 20 c/s. ringing tone to a rectifier unit 105, the output of which operates a relay 108 so as to energize a V.F. oscillator 106 and connect it to line. At the receiving end, this V.F. tone passes into the discriminating receiver 104 which, by operating relay 107, connects the local ringing supply from a mechanical vibrator 103 giving 20 c/s. to the indicator 101. The circuit of the receiver 104 is such as to discriminate against random tones of the same frequency as the ringing tone. As shown in Fig. 3 (Cancelled), the vibrator is fed from the smoothed output of transformer 304 and rectifier 303 and it feeds output terminals 309 through transformer 306. The frequency of the valve oscillator 106 is determined by a tuned transformer 310, Fig. 3 (Cancelled), connected between the anode and grid of the valve V5 and may be adjusted at 500 or- 1000 c/s. by means of taps on the transformer. Oscillation is suppressed, when no ringing tone is required, by earthing the screen grid at contact k of relay 108, this contact being opened as soon as the ringing key is operated. In the form shown, the tone of the oscillator is modulated by the 20 c/s. vibrator by taking a grid connection to the transformer 306. The output of the oscillator passes through a selector switch 351 which may select either an amplification of +6db or zero gain. A suitable power supply unit is described with reference to Fig. 5 (Cancelled) (not shown), and the circuit diagram of the complete apparatus except for the power supply is given in Fig. 6 (Cancelled) (not shown). This subjectmatter does not appear in the Specification as accepted.